This invention relates to r.f. couplers and in particular to coupling means for transferring microwave power from a coaxial line input onto a sliding coaxial line output without excessive loss of r.f. power and without the use of wiping contacts.
Situations exist in which it would be convenient and even vital to radiate microwave signals, generated aboard an aircraft, from a point several hundred to one thousand feet aft of the aircraft. In order to get the r.f. signal back to the desired locus of origin a long retractable conductor of convenient dimensions and low drag characteristics is required. The attenuation penalty of a one-half inch cable is greater than 50 db/1000 foot at S band, rising to greater than 0.1 db/foot at X band. This represents a prohibitive loss. Accordingly, there exists the need for a guiding conducting media with losses of less than 10 db/1000 feet. One presently available suitable conducting means is the so-called G-line, the single-wire transmission line described by G. J. E. Goubau in the publication entitled Single Conductor Surface Wave Transmission Lines, Proceedings of the IRE Vol. 89, P619, June 1951.
A problem arises in an operational situation in feeding such a transmission line as it rolls off a storage reel. Continuous r.f. transmission is needed for all positions of the G-line, and reel from the retracted to the fully extended position. For this purpose a coupler for coupling between a stationary launching means and a movable surface waveguide is needed.
In the laboratory and in fixed point to point type installations such as telephone lines, the feed coaxial cable is smoothly and rigidly coupled to the launch horn. In an attempt to accommodate an r.f. feed to an extensible transmission line the coaxial cable-to-stub 90.degree. transition described by Ragan in the MIT Radiation Laboratory Series Volume 9, page 181 has been built with an interior adaptation to permit a sliding conductor in the output ports. However, because the conductor is insulated (a necessary condition for G-line propagation) r.f. enters and escapes through the fine circumferential crack between the transmission line and the coupling device. This not only wastes r.f. power, but if uncontrolled leakage occurs the stated desire to have all of the r.f. energy originate from the tail end of the travel line is violated.
A coaxial-slot surface wave launcher structure has been developed and described in the periodical article entitled Coaxial-Slot Surface Wave Launchers by Beal and Dewar Electronics Letters 4:25, Dec. 13, 1968, pp 557-559. This device, however, is basically narrow band and cannot accommodate the sliding G-line in the various applications comprehended herein.
The foregoing summary of the state-of-the-art indicates that there currently exists the need for a coupling means that is capable of coupling r.f. signals from a fixed source to a sliding or extensible transmission line without excessive losses or bandwidth restrictions and without resorting to wiper contacts and the like. The present invention is directed toward satisfying that need.